Large diameter corrugated plastic (typically formed from polyethylene) pipe sections are known products. Past and present attempts to design water tight joints for corrugated plastic pipe to expand the use of the pipe to drainage and sanitary sewer applications typically involve radial compression of a gasket (molded or extruded and spliced) that occurs when an end section of the pipe formed as a spigot is inserted into a corresponding end section of a pipe formed into a bell. Pushing a spigot, with a gasket wrapped around its exterior cylindrical surface, into a bell having a tapered internal surface and interference fit, typically provides the radial compression of the gasket. The motion of pushing the gasket on the spigot in an axial direction and the frictional force on the interior lateral surface of the bell may cause the gasket to un-seat, twist and roll. In addition, circumferential tensile stresses on the interior of the bell, required to maintain gasket compression, decrease the stress crack resistance of the plastic pipe. A gasket is typically located in either a groove on the crest or crown or in the root of a corrugation near the pipe end of the pipe section. In the prior case the stiffness of the corrugation is not sufficient to withstand the hydrostatic water pressure required to prevent water leaks. As a result the corrugations supporting the groove that locates the gasket are sometimes made stronger by injecting rigid foam inside the corrugation. The latter case requires banding or other strengthening to avoid the large stresses that cause stress cracking. Both solutions add considerable cost obtain a reliable water tight joint.
A further disadvantage of joints that utilize radial compressed gaskets is that the joints are subject to failure, leaking water under pressure, as a result of rolling and twisting of the gasket or by stress cracking of the bell. William C. Andrick in U.S. Pat. No. 5,687,976 describes a symmetrical gasket for pipe joints that seals against both internal lateral surface of a bell and external lateral surface of a pipe having transverse corrugations. James B. Goddard in U.S. Pat. No. 5,765,880 discloses a bell and spigot joint for helical double wall corrugated pipe. Kenichi Hattori in U.S. Pat. No. 4,871,198 discloses a clamp type corrugated plastic pipe coupling in which a rubber gasket is utilized in radial compression to seal the joint. In the latter case, the metal clamp reduces the stress-cracking problem. However, the gasket transfers stress to the plastic inner wall or liner and to the corrugations. Both the corrugations and the liner are relatively low in stiffness and have difficulty maintaining the radial compression of the gasket. The high creep behavior of semi-crystalline polymers such as high-density polyethylene is responsible for increased leaking due to aging. It may also be difficult to assemble a joint as described by Hattori when the pipe sections to be joined are located in an open trench. Another disadvantage is the relatively high cost associated with an extended cylindrical gasket and a metal clamp. Bell and spigot type joints have an additional disadvantage insofar as field cuts of sections of corrugated pipe are not possible.
My co-pending related applications for patent identified above, which are incorporated by reference as if set out in full, relate in detail the characteristics and properties of corrugated polyethylene/plastic pipe and problems encountered by the prior art in attempting to effect a water tight seal between adjacent pipe sections. In my co-pending applications for patent, I disclose, inter alia, a method and design that avoids both the rolling of the gasket and stress cracking by utilizing a portion of the end corrugation to provide a means for self-energizing a water tight seal for joining two abutting sections of pipe.
Typically, soil tight performance in corrugated plastic drainage pipe is obtained by a compressing a gasket having a relatively small cross section when compared to the height and longitudinal width of a transverse corrugation. A transverse ridge at the crown of the end corrugation (referred to as a spigot) typically supports a joint gasket. The locating seat (transverse ridge) on the end corrugation has a disadvantage in that formation of the spigot requires a thermoforming mold to be introduced in the corrugation process and the other end of the corrugated plastic pipe either requires a molded bell at the end or necessitates the use of a dual bell coupler. This approach suffers from the weakness of the walls the corrugation supporting the crown of the corrugation and the gasket plus the cost associated with manufacturing in-line bell and spigot couplers. The present invention has the advantages of eliminating the requirement to introduce a bell and spigot mold pair that avoids the cost of the bell and spigot mold and of increasing the production rate of the pipe by removing the need to reduce production rates while forming of the bell and spigot.